System for motion control, method of using the system for motion control, and computer readable instructions for use with the system for motion control

ABSTRACT

A motion control system and method that includes a central controller configured to generate first and second demand control signals to be used to define actuation motion of respective first and second actuators. The central controller is in communication with first and second nodes by way of a data network, each node including at least a respective actuator configured to implement at an actuator time a motion or force-related effort based upon the respective demand control signal. Each node also includes a memory configured to store at least one respective propagation delay parameter related to a signal propagation delay between the central controller and the node. A timing mechanism establishes timing at each node based on the respective propagation delay parameter so that the actuator time at the nodes occurs simultaneously. Strictly cyclic and/or full-duplex high-speed communication can be supported. The network can be wired in a ring or as a tree and with twisted pair cabling or fiber. The central controller issues demand signals to the nodes that are actuator, servo motor drive, and/or I/O devices. The central controller can also provide a timing message that is used by the nodes, in conjunction with local delay correction circuitry, so that the simultaneous acquisition of data and the simultaneous implementation of controlled action occur.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/120,207, filed May 3, 2005, which is a continuation of U.S. Pat. No.7,024,257, issued Apr. 4, 2006, the entire contents of each of which arehereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the control of motion of complex machines, andspecifically to an improved system, method, and computer-readableinstructions designed to minimize wiring yet retain the flexibility ofcentralized control systems.

2. Discussion of the Background

The control of moving components in modern manufacturing equipment, suchas the equipment used to process and package semiconductor devices, hasbecome extremely complex as demands for both speed and precisionincrease. Such complexity is seen, for example, in the implementation offeedback control loops used in servo motion control systems. These servocontrol systems are used for critical positioning actuators, typicallyreferred to as motion axes, in a complex machine.

A schematic representation of an exemplary basic servo axis motioncontrol system 10 is shown in FIG. 1. Typically, a desired motion pathprofile is generated at a motion planning device 12, either prior tooperation or in response to a system stimulus. The desired motionprofile is converted to a motor signal (or other actuator signal) by thedrive 16. The drive 16 is typically a combination analog or pulse widthmodulated (PWM) amplifier along with the digital circuitry needed toconvert a digital motion profile into the analog actuator drive signals.

The desired motion path profile can be described and regulated inseveral ways. For example, a “demanded position” is the desired motionprofile expressed in terms of the position of the actuator (and/oractuated object). Alternatively, the “demanded velocity” of the actuatorand/or actuated object may be used, as well as a combination of velocityand position. A position measuring device, typically an optical motionencoder, is used to measure the “actual motion path profile” (e.g., the“actual position” and/or “actual velocity”) of either the shaft of themotor 15, or for more precise systems, the actual moving part position.

Due to various mechanical and electrical sources of error (such as,e.g., noise, backlash, part flexing), a difference between the desiredmotion profile and the actual motion profile is often present. Forexample, PID (Proportional Integral Differential) feedback 120 in theposition/velocity loop can be used to adjust the drive signal tominimize the error between demand and actual position. The algorithms inmany high performance servo motion systems go well beyond PID control.Servo systems often include filters to compensate for mechanicalresonances, tailored acceleration/deceleration profiles, and othersoftware intensive control elements that help optimize the speed andaccuracy of a moving system.

The control signal input to the drive 16 is usually a voltage, whereasmost motors generate a torque that is proportional to an input current.Therefore, the drive 16 must often convert the input voltagerepresenting the desired position and/or velocity to a currentcorresponding to the motor torque. This voltage-to-current conversionrequires another feedback element, the current loop 110, which may rangefrom a simple wire between the input and output of a linear amplifier tofairly complex digital logic controls in the case of PWM amplifiers.

Modern motion control systems may include several elements (e.g.actuators) that need to operate simultaneously to perform complex tasks.In other words, a motion control system may include several servo motors18 (or other actuators) and therefore several axes of control. Thedecisions about when and where to move using which trajectory are termed“motion planning.” The motion control system also includes a cascadecontrol scheme for each axis, where the elements in the cascade includethe current/torque control loop, and the position and/or velocitycontrol loop, which may either be combined or separate parts of thecascade.

Several types of digital networks have been indicated for use in motioncontrol. These networks can be divided up into two groups: those thathave been designed for general purpose data transmission but thenapplied to motion control and those that are designed specifically foruse as motion control networks.

1. General Purpose Networks

These are generally networks designed for data communications ormulti-media applications and adapted to machine control. Datacommunications networks are directed to moving data from point to pointin an efficient and reliable manner but they pay little or no attentionto the timeliness of delivery. For example, multi-media networks streamdata at 8 kHz from a CD player to an audio amplifier. Althoughmulti-media networks guarantee the average streaming rate, they do notprovide strict periodicity, as the data is buffered at both ends inmulti-media appliances and can therefore tolerate deviations in thedelivery period. Thus, multi-media networks are not ideal for real-timemotion control networks.

ARCNET

ARCNET is such a data communications network. Several companies areknown to have motion control networks based on ARCNET (ATA/ANSI 878.1, 2and 3). ARCNET is a medium speed token-bus network that can be operatedto provide strict cyclic messaging. However, there are limitations tousing ARCNET. The first limitation is a moderate bit rate (10 Mbit/s)that is not fast enough to send out torque/current demands. Moreover,the speed of ARCNET cannot be upgraded to 100 Mbit/s over copper withouta fundamental re-design.

ARCNET can use twisted pair copper but has no scrambling and thereforedata transmission may potentially cause electromagnetic interference.ARCNET can operate as a physical ring (or as a bus) but cannot operateafter a link has been severed. Nor can it report where such a fault hasoccurred, thereby impeding automatic and remote fault diagnosis. ARCNEThas no mechanism for auto-enumeration, which increases the timenecessary to commission a machine. ARCNET, being half-duplex, cannotcarry command and feedback data simultaneously. ARCNET uses atoken-passing scheme; such schemes are inherently less robust in theevent of electromagnetic interference than time-division multiplexing asthe loss of the token requires the network to re-start a configurationprocess wherein the token is re-created. ARCNET has no propagation timecorrection mechanism and therefore would suffer significant data skew ifa moderate number of nodes is used with a short cycle time.

IEEE Std 1394-1995 and prEN 1394b

This network is intended for multi-media applications and has a treetopology. The proposed 1394b version is to have both fiber and twistedpair copper layers, although neither has been reduced to practice todate. This network is proposed to operate at 100 Mbit/s or faster.

1394 is proposed to be cyclic, but not strictly cyclic. It is proposedto have a guaranteed rate for cyclic data but will delay transmission ofthe cyclic data depending upon network traffic. 1394 as proposed isrestricted to an 8 kHz cycle rate, but rates of 16 kHz or more arerequired to update the torque/current demand in a motion control system.1394b is proposed as full-duplex at the physical and data link layersbut only half-duplex at the application layer. In other words, commandand feedback data cannot be sent simultaneously. The proposed 1394cannot operate as a ring and cannot operate all nodes in the event of afault such as a broken cable. The proposed 1394 has auto-enumeration andnode discovery but unfortunately these functions are ‘hot-pluggable’(i.e. the network is re-enumerated when a new node is plugged-in orremoved). Therefore, the position of a node within the network cannot bereliably used to signify the axis of control since this position wouldchange if another node were disconnected during operation. Proposed 1394has no time-correction mechanism for dealing with the situation where asynchronization message is received at different times at differentpoints in the network, and therefore would suffer significant data skewif a moderate number of nodes were used with a short cycle time.Existing 1394 and proposed 1394b can, however, correct for a delay insending a synchronization message.

Ethernet-IEEE 802.3-1998

This general network is intended for data communications applicationsand has a tiered hub topology that operates at 100 Mbit/s and above. Thepoint-to-point physical layer can be used to form the network in oneembodiment of the present invention.

Ethernet has no inherent cyclic messaging. Ethernet also cannot beoperated as a full-duplex ring and therefore has no means of operatingall nodes after a link has been severed. No known implementation of anEthernet network can report where such a fault has occurred, therebyautomatic and remote fault diagnosis is prevented. Ethernet lacks anappropriate mechanism for auto-enumeration since it cannot assign nodenumbers on the basis of position. Ethernet is normally half-duplex;full-duplex operation introduces the expense, complexity, and temporaluncertainty of using switches. Ethernet lacks a time-correctionmechanism.

2. Networks Designed Specifically for Motion Control

The technical details of many of these networks are proprietary, andonly some have been disclosed to the public. Only two of these networkspossess a bus speed of exceeding 10 Mbit/s as is required to meet thefairly modest objective of achieving an update rate of 8 kHz on fouraxes with a telegram size of only 19 bytes.

SERCOS

The concept of cyclic, time-domain multiplexed messaging originated withthe SERCOS network and was subsequently codified as IEC 61491. SERCOS isimplemented upon a single fiber optic ring, is half-duplex, and canoperate at up to 16 Mbit/s. While this network is accepted for use incertain industry sectors such as machine tools and automotive assemblylines, it has several significant limitations.

In a network with more than a few nodes, the SERCOS bit rate of up to 16Mbit/s substantially limits the network cycle time to a moderate 1 ms.This moderate network cycle time in turn dictates that the positionfeedback loop must be closed at the drive rather than centrally at themotion controller. SERCOS networks thus limit the feasibility of closeinter-axis coupling (including centralized implementation of electronicgearboxes and cams) and on-the fly changes of trajectory. The moderatebit rate also limits the number of axes that may be attached to anetwork to around 16 when there are many applications that demand morethan 16 axes. SERCOS does not work with a copper transmission medium,which significantly increases the cost and poses a problem inapplications where the network medium must be routed through flexibletrunking such as cable retractors. This is due to the fact that the longterm flexure characteristics of fiber are poor. SERCOS furthermore doesnot operate after a link has been severed, nor does it report where afault has occurred. Finally, SERCOS has no mechanism forauto-enumeration and relies on DIP switches to identify nodes.

MACRO

MACRO is a proprietary network introduced by Delta-Tau Inc. It uses theFDDI fiber-optic physical layer to transmit 100 Mbit data on a singleoptical ring. MACRO has a weak error detection mechanism based on achecksum—not a CRC. MACRO, according the March 1999 specification has nolong-haul (100 m) copper medium. MACRO cannot operate in a tree topologyor in the event of a fault and lacks node discovery and auto-enumerationcapabilities. It is unclear whether MACRO has a time-correctionmechanism and therefore may suffer significant data skew if an evenmoderate number of nodes were used with a short cycle time.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide a system formotion control, a method of using a system for motion control, and acomputer readable medium storing instructions for operating a system formotion control that provides a motion control system with a reduced needfor wiring while retaining centralized control of the motion system.

These and other objects of this invention are achieved according to thepresent invention by providing a novel system, a method of using thesystem, and a computer program product storing instructions forimplementing a system for motion control. This system for motion controlincludes a central controller configured to generate first and seconddemand control signals defining actuation motion of respective first andsecond actuators; first and second nodes in communication with thecentral controller, each node including, at least a respective one ofthe actuators configured to implement at an actuator time an actionbased upon the respective demand control signal, and a memory configuredto store at least one respective propagation delay parameter related toa signal propagation delay between the central controller and the node;a timing mechanism configured to establish timing at each node based onthe respective propagation delay parameter so that the actuator times atthe nodes occur simultaneously; and a data network configured to placethe first and second nodes in communication with the central controller.

A node configured to be in communication with a central controller overa data network can thus include a delay correction memory configured tostore a delay correction signal related to a propagation delay betweenthe central controller and the node over the data network; and anactuator configured to implement a motion or force-related effort at anactuator time based upon the stored delay correction signal. A nodeconfigured to be in communication during operation with a centralcontroller over a full-duplex data network can alternatively include afirst receiver configured to receive a first signal transmitted along afirst direction over the data network; a second receiver configured toreceive a second signal transmitted along a second direction over thedata network; and an actuator configured to implement an action basedone of the received first signal and the second signal.

The operations of a plurality of actuators in a system for motioncontrol can thus be synchronized by determining a respective propagationdelay between a central controller and each actuator of the plurality ofactuators; and timing operations of each actuator of the plurality ofactuators based on the determined respective propagation delay for eachactuator.

A plurality of nodes controlled by a central controller in a system formotion control can be autoenumerated by linking the central controllerand the nodes in a network; transmitting a query message from thecentral controller in along the network; receiving the query message ata node of the plurality of nodes in the network; transmitting an answermessage to the query message along the network from the node to thecentral controller, the answer message enumerating the node; receivingthe answer message at the central controller; retransmitting the querymessage from the central controller along the network; relaying theretransmitted query message through the enumerated node to a furthernode; and transmitting a further answer message to the query messagealong the network from the further node through the enumerated node tothe central controller, the further answer message enumerating thefurther node.

Moreover, the operations of a plurality of nodes in a ring networkcontrolled by a central controller can be maintained in the event of afault by transmitting first messages addressed to plural respectivenodes in a first direction along said ring network; monitoring firstreply messages transmitted by said plural nodes in a second directionalong said ring network in response to the transmitted first messages;identifying, when first reply messages are not received from each nodeto which a first message was transmitted, a first subset of nodes fromwhich said first reply messages are received, and based thereon,determining a second subset of nodes exclusive of said first subset ofnodes, from which respective first reply messages were not received;transmitting in said second direction, when first reply messages are notreceived from each node to which a first message was transmitted, secondmessages addressed to respective nodes in said second subset of nodes;receiving second reply messages transmitted by respective of said nodesof said second subset of nodes in response to said second messages, saidsecond reply messages traveling in said first direction along said ringnetwork; transmitting in said first direction third messages addressedto said first subset of nodes; and transmitting in said second directionfourth messages addressed to said second subset of nodes.

Finally, a computer readable medium storing instructions for executionon a computer system, which when executed by a computer system, cancause the computer system to synchronize the operations of a pluralityof actuators in a system for motion control, autoenumerate a pluralityof nodes controlled by a central controller in a system for motioncontrol, or maintain operations in the event of a fault of a pluralityof nodes in a ring network controlled by a central controller asdescribed above.

Further objects and advantages of our invention will become apparentfrom a consideration of the drawings and the technical description.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same become betterunderstood by reference to the following detailed description whenconsidered in light of the following drawings, wherein:

FIG. 1 illustrates an exemplary one-axis motion control system;

FIG. 2 illustrates exemplary layers of the cascade control of a two-axismotion control system variation of FIG. 1;

FIG. 3 illustrates an exemplary system cabled in the ring topology;

FIG. 4 illustrates an example format of a network telegram;

FIG. 5 illustrates examples of different types of network telegrams intabular format;

FIG. 6 illustrates an exemplary state diagram of a system for motioncontrol;

FIG. 7 illustrates a simple exemplary undiscovered ring network;

FIG. 8 illustrates the transmission of an exemplary master querytelegram during an exemplary network discovery;

FIG. 9 illustrates the transmission of an exemplary answer telegramduring an exemplary network discovery;

FIG. 10 illustrates the transmission of an exemplary master assignmenttelegram during an exemplary network discovery;

FIG. 11 illustrates the transmission of an exemplary second answertelegram during an exemplary network discovery;

FIG. 12 illustrates the transmission of an exemplary port telegramduring an exemplary network discovery;

FIG. 13 illustrates accessing of the next node in the chain with asecond master query telegram during an exemplary network discovery;

FIG. 14 illustrates an exemplary timing process for determining anoutward ring delay during an exemplary network discovery;

FIG. 15 illustrates an exemplary process of setting a RPTC of a firstnode during an exemplary network discovery;

FIG. 16 illustrates an exemplary confirmation of an RPTC by a first nodeduring an exemplary network discovery;

FIG. 17 illustrates the transmission of an exemplary master querytelegram in the reverse direction during an exemplary network discovery;

FIG .18 illustrates an exemplary format of a Master Synch Telegram;

FIG. 19 illustrates an exemplary format of a Master Data Telegram;

FIG. 20 illustrates an exemplary format of an Amplifier Telegram;

FIG. 21 illustrates examples of outward messaging on a ring network thatoccurs during the operational phase;

FIG. 22 illustrates examples of inbound messaging on a ring network thatoccurs during the operational phase;

FIG. 23 illustrates examples of outbound messaging on a ring networkthat occurs under fault conditions during the operational phase;

FIG. 24 illustrates examples of inbound messaging on a ring network thatoccurs under fault conditions during the operational phase;

FIG. 25 illustrates an exemplary approach to network timing under faultconditions;

FIG. 26 illustrates a simple exemplary ring network;

FIG. 27 illustrates a timing diagram describing the synchronousoperation of multiple nodes;

FIG. 28 illustrates another simple exemplary ring network used todescribe scheduling (strict timing and free timing);

FIG. 29 illustrates outbound messaging in a tree network during theoperational phase; and

FIG. 30 illustrates inbound messaging in a tree network during theoperational phase.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numeral designateidentical or corresponding parts throughout the several views, and moreparticularly to FIG. 2 thereof, wherein a generic two-axis controlsystem with each element of the cascade is illustrated. Each of theregions 2A, 2B, 2C, 2D, and 2E represent elements of the control system.Namely, 2A illustrates an exemplary system host and man/machineinterface, 2B illustrates a motion planning device, 2C illustrates anexemplary position control element, 2D illustrates an exemplary velocitycontrol element (including a differentiator 24 on the feedback line), 2Eillustrates an exemplary current/torque control element, and 2Fillustrates an exemplary drive unit which may include, for example, asignal amplifier 26, the motor/actuator 27, and position, velocity,and/or acceleration transduction elements 28. Although the feedback toeach of the control elements is shown as including a PID (ProportionalIntegral Differential) control 25, it is to be understood that variousforms of digital compensation elements may be used. The actual digitalcompensation elements used will depend upon the position of the controlelement in the cascade and the control system application.

Moreover, some motion control systems operate by providing a series ofdiscrete signals to the actuators. Examples of such signals are Step andDirection and Clockwise/Counterclockwise. In these systems, either noclosed loop feedback is required, or the closed loop control is in thedrive and the drive is controlled by discrete signals from the motioncontroller. These systems still have many of the elements of thecascade. Although the example of servo control systems is used for thisinvention, closed loop feedback is not an integral part of theinvention, and the inclusion of discrete actuators using the inventionis explicitly included.

Although each of the elements of the cascade are illustrated as discreteunits in FIG. 2, modern implementations of control systems often combineone or more of these units. For example, as described in regard to FIG.1, position 2C and velocity 2D control can be combined in a singlecontroller. As a further example, the most common implementation of amotion control system is a “motion controller” that includes a programrunning on a central processor unit that, in some embodiments, mayitself be in communication with a system host (such as a personalcomputer) that also supervises other machine control functions such as avision system, a display, etc. for motion planning. In these systems,not only will all of the motion planning be carried out in the “motioncontroller,” but at least a part of the cascade control structure willbe implemented at the motion controller as well. The part of the cascadecontrol implemented at the motion controller commonly includes eitherjust the position control loop or both the position and velocity controlloops, indicated in FIG. 2 as 2C and 2D, respectively. The highestperformance existing motion controllers are formed by a dedicated motioncontrol processor printed circuit card, with an associated motionprogramming interface, residing on the bus of a system host. Thecombination host/board level motion controller performs all of the tasksillustrated as units 2A, 2B, 2C, and 2D in FIG. 2. Such a motioncontroller is commonly referred to as a centralized motion controlsystem because all motion planning decisions are taken centrally, alldata is collected centrally and almost all control loops are closedcentrally. Either of the units 2C or 2D of FIG. 2 thus represents theextent of the cascade control that is implemented at the motioncontroller. In a real centralized motion control system, the controlsignals are analogue or pulse demand signals that are sent from themotion controller to the drive, and position feedback signals arereturned from the transducer 28 on the motor 27 (possibly routed via thedrive).

Although these serve as the fundamental operational platforms of motioncontrol systems in many thousands of operating machines, it isrecognized that these platforms have many shortcomings. Firstly, thecost of the cabling (demand cable, feedback cable, and I/O cable plusconnectors—in the whole amounting to 14 to 20 or more terminations), isquite high. The installation of the cabling, especially where thecontrol signals for multiple axes are to be routed through a drag chainor cable retractor, is quite difficult and/or bulky. For example, dragchains are typically used when signals must be routed to a movingcontrol element, such as the machine head suspended on an XY stage. Thedrag chain is thus often moved at high speeds. If the machine head(pick/place head, etc.) uses servo motors, the drag chain will have alarge number of wires and will be quite inflexible and large.Furthermore, these motion control systems do not provide a mechanism forgathering data other than the transducer (position) feedback signal fromthe amplifier or motor. As yet another example of a problem in motioncontrol systems, the distance between the motion controller and thedrive is limited in practical terms.

For these and other reasons, there is a growing tendency towards theadoption of digital networks in motion control systems. A digitalnetwork, properly implemented, eliminates the need to route the 14-20 ormore lines per axis from the motor/drive/position sensor to the host.Rather, one thin two-pair network cable connects all local control nodesto the host. The use of local nodes allows the 14-20 or more lines to belimited to the area immediately surrounding the motor. The reduction inwiring count and complexity is thus quite dramatic. Such a system iscommonly referred to as a “distributed motion control system” in theliterature although the term “networked control system” might be moreaccurate since networks can be used either to concentrate controlfunctions at the center or to distribute them to the nodes.

Distributed motion control systems retain the fundamental cascadecontrol structure, but insert a two line, digital communication path(e.g., a network) between at least two of the elements 2A, 2B, 2C, 2D,and 2E in FIG. 2. In the design of distributed motion control systems,there are many options regarding the location of the network in thecascade control system. The various locations may all achieve the samereduction in wire count, but result in significant differences in systemdesign and performance.

The first common location of the network is between the units 2A and 2Bof FIG. 2. In this case units 2B, 2C, 2D, 2E and 2F are divided-up(horizontally in the diagram) on a per-axis basis into separatenetworked drive units. Employing the network at this location transfersall of the cascade control for each axis to the drive itself togetherwith the associated motion planning. The drives in the resulting controlsystem are called “smart drives,” and the network is employed to linkthe smart drives to the host (2A in FIG. 2). This arrangement can worksatisfactorily provided that the motion planning, and indeed the cascadecontrol, for each axis can be independent. In other words, there shouldbe little or no need for inter-axis coupling (necessary, for example, toperform coordinated multi-axis moves) to enable use of thisarchitecture. Since all of the high speed control loops are in thedrives, there are no feedback signals transmitted across the network anda moderate bandwidth network is adequate.

However, many machines and manufacturing processes require tightinter-axis coupling to, for example, implement coordinated multi-axismoves, realize electronic gearboxes, change trajectories on-the-fly, andachieve time-optimal moves. Examples of such machines include a pick andplace machine for electronic surface mounted components and machinesused to process and to package semiconductors. In such cases, not onlydoes the motion planning require simultaneous actions along more thanone axis but the cascade control system may also pass terms from oneaxis to another (not indicated in FIG. 2 for reasons of clarity). Hence,it is often essential to retain the flexibility of a central motioncontroller.

-   -   There are other problems with implementing a multi-axis motion        control system using multiple smart drives. A first problem is        that, by transferring extra responsibility to the drives, extra        hardware is added to each drive. This hardware includes, for        example, not only extra processor(s) and memory, but also other        features such as I/O and displays. Thus, on the system level,        much of this hardware and its functionality is unused and/or        redundant, but it still must be purchased and space must be        provided at the drive. A second problem, which is indicative of        recent developments in smart drives, is that it is necessary to        write and maintain separate programs for each smart drive.        Moreover, this must be done in the absence of a standardized        programming environment for all drives. No longer are the        familiar ‘C’ (or other high level programming environment)        compilers provided by the major vendors of board level        controllers available, and in their absence there is no        standardized method of communication with smart drives. Another        significant disadvantage is that smart drives are more difficult        to miniaturize than simple drives. This makes it difficult to        achieve the optimum wiring reduction possible for a distributed        motion system: no existing high performance smart drives are        small enough to mount on a pick/place head for fast SMT machines        and thus a smart drive would not lower the wire count in the        drag chain for this application.

The case for retaining the concentration of control functions at thecenter in a multi-axis control system is thus compelling. A problemremains in that it is difficult to retain this centralized model yetadopt a network connection to the drives in order to achieve the desiredwiring reduction because of the extra demands this places on thenetwork.

The second common location of the network is between the units 2C and 2Dor between the units 2D and 2E shown in FIG. 2. This location retainscentralization of control functions and uses the network solely toachieve wiring reduction and data gathering. Employing the network atthese locations places the network between the motion controller and thedrives along the control cascade. Since the network is digital, themotion control system is in fact a sampled data system. The motionplanner provides demands at a regular rate (typically a value in therange 1 to 4 kHz), the control loops in the cascade control systemoperate more quickly (typically 1 to 4 kHz for the position loop and 2to 8 kHz for the velocity loop) and the current loop operates at 16 kHzor faster.

It is desirable in a sampled data system for the various elements tooperate in synchronism so as to avoid the uncertainties that would beintroduced by variable transport delays and in a networked sampled datasystem this means that there must be a common network time reference.One known method of generating a common network time is as follows: themotion controller emits a timing reference message at regular intervalsand the receiving nodes then each use this signal to calibrate a localclock which is derived from a much higher local frequency source. Thislocal clock is used as the timing reference for the PWM and any localcontrol loops. Typically, the multiplication of the network timingreference is performed using phase locked loops. For this reason,strictly cyclic messaging is required since any jitter on the networktiming reference will propagate into all of the dependent local clocks.Moreover, in a physically large network (or one where the messages passthrough numerous repeaters), the propagation delay to the nodes may besignificant and will differ from one node to another and this canintroduce delays and therefore non-simultaneity into the node clocks.

FIG. 3 illustrates an exemplary network 30 for motion control accordingto the present invention that is cabled in full-duplex ring topology.The ring topology illustrated is cheaper than a tree because there is norequirement for a hub or extra ports at each node and is thus normallypreferred. The physical layer of this network for motion control 30 canbe based, e.g., upon known data transmission networks such as, forexample, IEEE 802.3-1998, Clauses 24-26. IEEE 802.3-1998, Clauses 24-26are in turn based upon ANSI X3.263-1995 and thus a choice of fiber orcopper transmission media is offered. Such a physical layer is generallyknown as 100BASE-X, but the wiring topology does not follow IEEE802.3-1998, Clauses 24-26.

The network for motion control 30 includes nodes (alternatively referredto as slaves) 32 and 34 of FIG. 3 that are connected by a network link 2to each other on one side and to the master (motion controller) 36 onthe other side by network links 1 and 3, as illustrated in FIG. 3.Further nodes can be added to the ring topology with further links. Theaddition of a further node will result in a node being connected only tothe other nodes, and not with the master 36. The network 30 in thisexemplary embodiment is strictly master-slave. The master 36 initiatesall communication. The slave nodes 32 and 34 respond to the master 36,and do not communicate with one another. The master 36 can alsobroadcast to one or more of the nodes 32 and 34.

Each of the nodes 32 and 34 and the master 36 includes a P0 port 310, aQ port 320, at least one PHY (“physical entity”) device 330, and a MAC(“medium access controller”) 340A, 340B, or 340C. PHY devices 330 areattached to a transmission/reception port on one side and to the MAC 340on the other. PHY devices 330 are part transceiver, part encoder/decoderand send and receive signals on a point-to-point link. PHY devices 330thus operate at the level of bits and symbols and are availableoff-the-shelf either as silicon or as IP cores conforming to theappropriate standards of the selected physical layer that forms the ringnetwork 30. The MAC's 340A, 340B, and 340C implement the digital logicresponsible for exchanging messages with (typically) a processor on oneside and the PHY 330 on the other. The MAC's 340A, 340B, and 340Coperate at the level of symbols and messages. The master MAC 340C andslave MAC's 340A and 340B have different functionality.

The MAC devices 340A and 340B in the slaves act either as the originatorof a message or they can repeat incoming messages. In the ring network30, a message received at the PHY 330 on the Q port 320 is repeatedthrough the PHY 330 at the P0 port 310 and a message received at the PHY330 on the P0 port 310 is repeated through the PHY 330 at the Q port320. The exemplary physical layer supports point-to-point links only,i.e., it cannot be wired as a bus and this is generally true of allphysical layers than run faster than around 20 Mbit. Therefore, messagesthat are to be received by a node beyond the end of a given link must beconveyed through a repeater and re-transmitted. Since the network isfull-duplex, a repeater is required for each direction.

The repeater function of the MAC devices 340A, 340B, and 340C isimplemented using a FIFO in each direction (not shown). Aside fromconveying telegrams to further nodes, the FIFO's achieve two otherobjectives. Firstly, they isolate the clock domain of the receiver fromthe local clock domain (which is also the transmitter clock). Secondly,they allow the inevitable slight differences in the transmit clock andreceive clock frequencies to be tolerated without loss of data.

The required depth of the FIFO depends upon the maximum discrepancybetween clock frequencies, the FIFO width (it is normally 4-bit wide,depending on the PHY devices) and the maximum packet size.

The final link in the ring network 30 is referred to as the return link3. The return link 3 is largely redundant for operational traffic sincethe network 30, being full-duplex, allows unrestricted communicationwith all slaves 32 and 34 via the P0 port of the master 36. However, thereturn link 3 to the Q port of the master 36 is needed to allow themaster 36 to verify that packets transmitted from the P0 port of themaster 36 travel through the complete ring network 30 to the Q port ofthe master 36. Similarly, packets transmitted from the Q port of themaster 36 will reach the P0 port of the master 36 via the ring network30. In the event of a fault condition it is possible to infer theposition of the fault by noting which messages fail to be received atthe P0 port 310 and Q port 320 of the master 36.

The network links 1 and 3, the P0 port 310 and the Q port 320 areoff-the shelf, commercially available items (PHYs, cabling, fibre,couplers/connectors). The PHY 330, the FIFO, and the MAC devices 340 canbe implemented in an FPGA or other digital logic such as an ASIC. Allfunctions including the automatic answer mechanism, the phase-lockedloop to create the local clock, the FPTC/RPTC time correction mechanism,the detection of a severed link by a timer watchdog and subsequent useof the MTT instead of the MST are digitally realized.

In such a network system 30, the master 36 (motion controller) canperform current/torque feedback control of a motor or actuator node 32and/or 34. For example, in a network system 30 for motion control with16 axes, a 16 kHz update rate with a total telegram size of 24 bytes ineach direction has a bit rate of at least 100 Mbit/s. This is within thecapabilities of the network system 30 described above.

The network communication within the ring network 30 is in the form ofshort packets sent out from the master (motion controller) 36 at regularintervals to provide the demand signals, and short packets sent backfrom the nodes 32 and 34 to the master 36 at regular intervals toprovide feedback and monitoring data. Not only must the regularintervals align with the control system loop rate at the point in thecontrol cascade where the network 30 is inserted but a network time mustbe established so that all demands take effect simultaneously and allfeedback is acquired simultaneously.

An example of a telegram 4 that would be transmitted from the master 36is illustrated in FIG. 4. The telegram includes an address field 42, acontrol field 43, a data field 44, and a frame check sequence 45. Thestart of stream 41 and end of stream 46 delimiters delineate thetelegram.

Each address field 42 can be used to identify the destination node (suchas slave nodes 32 and 34 of FIG. 2) for the data in the telegram. Eachslave node recognizes at least one slave address, namely the baseaddress for that node. In some embodiments, a node may include more thanone control entity such as, for example, a servo axis. In these cases,there is an additional network address for each additional controlentity that is present at the slave node. The base address of a slavenode can be assigned topologically during the node discovery process andthe additional addresses for the additional control entities cansequentially follow the base address. For example, a network 30consisting of two four-entity nodes can be enumerated [1, 2, 3, 4] atthe first node and [5, 6, 7, 8] at the second node. In one embodiment,the address field is one byte long and its meaning (the address of thesource node or the address of the destination node) varies according tothe telegram type.

Each control field 43 can be used to indicate the type of telegram 4being transmitted to the receiving node. Furthermore, it can alsoindicate how the address is to be interpreted. FIG. 5 enumeratesexamples of different types of network telegrams in a tabular format.For the sake of convenience, a naming convention can be adopted asillustrated in FIG. 5, where telegrams originating from the master startwith M and those from the slaves start with A for amplifier. Differenttelegram types can also be associated with different operational states(otherwise referred to as phases) of the network for motion control.Naturally, other telegram types that perform additional functions orcombinations of the functions described below can be added to extend thecapabilities of the network, and this would be denoted in the controlfield 43. The specific telegram types illustrated in FIG. 5 are providedsolely to further clarify some of the communication capabilities of thenetwork according to the present invention.

Each data field 44 of the telegram 4 has a variable length and is usedto communicate data between the master 36 and the nodes. This data caninclude, for example, actuator or actuated object position informationfor motion control, and the drive signals themselves. Given the state ofthe art in current physical layers, there are preferred limits on thelength of the data fields 44, although it will be obvious to thoseskilled in the art that larger data fields 44 can be used as equipmentis developed. The current limits are determined by, e.g., the efficacyof the frame check sequence and the depth of repeater FIFO's, as well asother economic and operational considerations. In a typical embodiment,2048 bytes is considered practicable at the present time.

The frame check sequence 45 is used to identify errors in the telegram 4that typically arise due to equipment malfunction and/or electricalinterference. A frame check sequence suitable for use in the presentinvention is a 32-bit CRC according to IEEE802.3-1998 Clause 3.2.8,namelyG(x)=x ³² +x ²⁶ +x ²³ +x ²² +x ¹⁶ +x ¹² +x ¹¹ +x ¹⁰ +x ⁸ +x ⁷ +x ⁵ +x ⁴+x ² +x+1since this sequence is known to be compatible with the scramblerpolynomial in the physical layer.

As illustrated in the state diagram of FIG. 6, the network operation hastwo phases (or states), namely the discovery phase 60 and theoperational phase 62.

The discovery phase 60 describes operations whereby the nodes (such as,e.g., node 32 and 24 of FIG. 3) are discovered and enumerated (e.g.,assigned address and other network parameters) by the master 36.Communication during the discovery phase 60 is asynchronous and theslave normally sends a telegram in response to one received from themaster 36.

On the other hand, the operational phase 62 is characterized by cycliccommunication. For example, the master sends a timing telegram (e.g.,the MST of FIG. 5) followed by the telegrams conveying theposition/velocity/torque/current demands (MDT's) and the slave sendsback status and feedback data as amplifier telegrams (ATs).

The exact time at which the telegrams such as the ATs and MDT's are sentmust meet certain criteria for correct operation of the physical layer.However the relationship between the two can be freely chosen to meetapplication needs; for example the MDT could be sent before, after, orsimultaneously with the AT for a given node. In one embodiment, all ATsand MDT's must be completed before the next MST so that timing can beupdated regularly.

The discovery phase 60 always occurs prior to the operational phase 62,although an operational phase 62 can be interrupted to return thenetwork to a discovery phase 60. Slave nodes are in the undiscoveredstate immediately following power-up. They can also be forced back intothis state along path 600 by the master 36 sending out a hide packet(the MHT), after discovery of a fault, for example, at which time theslaves then propagate this packet throughout the network. A simplenetwork ring 30 prior to the discovery process after power-up isillustrated in FIG. 7. At first the slaves 32 and 24 are undiscovered bythe master 36 following power-up and are hence shown in dashed lines. Inother words, the master 36 has not identified and/or enumerated theslaves 32 and 34. At this time, the master issues an MQT (master querytelegram, illustrated in FIG. 8) from its P0 port over network link 1,as illustrated in FIG. 8. The first slave 34 in the network receives theMQT but, since slave 34 is undiscovered, slave 34 does not pass the MQTalong. Instead, slave 34 replies with an AAT (amplifier answer telegram,illustrated in FIG. 9) that supplies the master 36 with certaininformation about the slave 34. This information is commonly theinformation that is required for operation of the network and wouldinclude, e.g., the number of slave addresses required by the node, thenumber and type of control entities, etc.

The delay between the transmission of the MQT the master 36 and thereception of the AAT by the slave 34 is repeatable to an uncertaintyassociated with the physical layer clocks. Hence by timing the delaybetween transmission of the MQT and reception of the AAT and thensubtracting the known internal delay of the slave 34 when transmittingthe MQT, the propagation delay caused by the cabling and the repeatersin any intervening nodes may be computed by automated means. The knowninternal delay of the slave 34 has two elements. The first arises due tothe FIFO's of the slave 34. This is a fixed quantity, dependent ondepth. The second is the time taken for the node to automaticallyrespond to the MQT with an AAT. This is a fixed number of clock cyclesfor fully synchronous circuitry. Thus, the sole elements of uncertaintyare the resolution of the governing clocks (master and slave) and too alesser degree the drift and accuracy thereof. Nevertheless, this knowninternal delay is then used to calculate a forward path time correctionfactor (FPTC) for the particular node (such as node 32 in FIG. 8 and 9).After a determination of the FPTC, the master then issues a MAT (masterassignment telegram, as illustrated in FIG. 10) from its P0 port. TheMAT thus assigns the lowest available network address to the slave 34and sets the FPTC for slave 34. The slave 34 responds to the MAT with afurther AAT (as illustrated in FIG. 11) to confirm that the addressassigned by the MAT has been accepted by the slave 32. In FIG. 11 and inall further illustrations, slave 34 will be denoted as slave 34E toindicate that it has been enumerated.

After enumeration of slave 34E, the master 36 issues an MPT (master porttelegram, as illustrated in FIG. 12) from its P0 port that activates theP0 port on slave 34E. This causes slave 34E to relay messages though itsinternal FIFOs, and thus it becomes possible to discover and enumeratefurther nodes on the ring network such as slave 32. This discovery andenumeration process proceeds for slave 32 as it did for slave 34E. Itshould be noted that when the MQT is transmitted through slave 34E toslave 32 (as illustrated in FIG. 13), the FPTC will include the delaycaused by the repeaters of slave 34E. In all further illustrations,slave 32 will be denoted as slave 32E to indicate that it has beenenumerated. Naturally, the discovery, enumeration, and configuration canbe extended to more slave nodes than herein illustrated. For the sake ofbrevity and clarity in the figures, further slave nodes have beenomitted.

In one embodiment, when all of the slaves in the ring have beendiscovered, enumerated, and configured, it is determined if the ring iscomplete and the total time taken for the MQT to travel from the P0 portof the master 36 to Q port of the master 36 is measured (as illustratedin FIG. 14). The total delay time is then used by the master 36 tocompute the delay of the return link 3. The master 36 can store thepropagation delay to every node in the network, including thepropagation delay of the final link 3. Armed with this information, themaster 36 then computes the delay from the Q port of the master 36 toeach node in the network. The delay from the Q port of the master 36 toeach node is used to calculate the reverse path correction time (RPTC)in a manner similar to that described with regard to the FPCT. Themaster 36 can then assign the RPTC for each node to each node using theMCT (master configuration telegram) as illustrated in FIG. 15 with theaddress set to target a particular node, and receives answers confirmingthe assignment as illustrated in FIG. 16.

In an alternative embodiment, the discovery, enumeration, andconfiguration process can be repeated by instructing all the nodes tohide using an MHT, and then repeating the above-described process usingthe Q port of the master 36. For example, the first step of this processwould include transmitting a MQT from the Q port of the master 36 and(re)discovering node 34 as illustrated in FIG. 17. The propagationdelays in the forward and reverse directions can be compared to helpensure the operability of the network and all of the slave noderepeaters. Both the forward and reverse propagation delays can bestored, averaged, and/or combined as needed.

After the discovery phase 60 is complete, the network for motion controlcan then enter the operational phase 62. This can be indicated, forexample, by transmission of a MST (master synchronization telegram) fromthe master 36 over the network. The principal purpose of the MST is toprovide a timing reference for the slaves, and an example embodiment ofone MST is illustrated in FIG. 18. Another way of indicating a timingreference to the slaves includes transmission of a MTT (master timingtelegram), which indicates the equivalent of a MST in to some nodes inring network topologies in the event of a severed link or similarnetwork fault.

The MST illustrated in FIG. 18 includes an address field 172, a controlfield 174, a data field 176, and a FCS field 178. These fieldscorrespond to the address field 42, a control field 43, a data field 44,and a frame check sequence 45 of FIG. 4, with the exceptions that theaddress field 172 indicates that the MST is to be “broadcast” (i.e.,received and used by all slave nodes) and the data field 176 in the MSTis empty. Rather than carry data, the MST simply indicates a commonpoint in time which the slave nodes can use to synchronize theirrespective clocks in light of the FPTC (and/or RPTC) set duringconfiguration.

In the embodiment of the operational phase 62 currently described,demands can be transmitted to exactly one slave using a MDT (masterdemand telegram). An exemplary MDT is illustrated in FIG. 19, where theaddress field 182 indicates a single slave node and the data field 176includes run-time demands for the single slave node indicated in theaddress field 182. In an M-slave network, there are M possible addressesfound in the address field 182, and each MDT can be designated as anMDT1, MDT2, or so forth for convenience sake. Naturally, a single MDTcan be addressed to plural or even all slave nodes simultaneously, by,e.g., defining one or more further addresses to indicate the desiredplurality of slave nodes.

Also in the operational phase, run-time data (e.g., the actuator and/oractuated object position for feedback control) can be returned to themaster 36 from a slave node using an AT (slave telegram). An exemplaryAT is illustrated in FIG. 20. These fields correspond to the fields ofthe exemplary telegram illustrated in FIG. 4, with the exception thatthe address field 192 indicates the address of the slave node that isthe source of the AT. Moreover, the data field 196 can include both nodestatistics 1961 and application data 1962, where node statistics relateto network information (such as, e.g., indicating whether the last MSTand MDT were received correctly), and the application data relates tothe motion operations of the slave node. In a preferred embodiment, theAT retrieves run-time data from exactly one slave and thus, in anM-slave network, there are M possible addresses found in the addressfield 192, and each AT can be designated as an AT1, AT2, or so forth forconvenience sake. Naturally, other AT's can include data from pluralslave nodes in a system where one slave communicates with another or ina tree topology.

One exemplary message routing scheme during normal operation isillustrated in FIG. 21. In the illustrated example, outbound telegramsMST and MDT first proceed from the PO port of the master 36 to the Qport of the slave 32. They are decoded by the dual PHY element, andtransferred to the MAC of the slave 32. In telegrams that are“broadcast” (such as a MST), every MAC will use the telegraminformation. In telegrams that are addressed to a specific slave node(such as a MDT), only that slave node will act upon the information inthe telegram. In a preferred embodiment, every telegram is routed aroundthe entire ring network, with each slave node repeating every telegram.Therefore, the master will have its own outbound messages returnedacross the final link after the messages are received and repeated byall of the slaves. This allows the master 36 to confirm the continuityof the ring network.

An exemplary routing scheme for inbound (directed to the master 36)telegrams is illustrated in FIG. 22. In the illustrated example, inboundtelegram AT is first transmitted from the Q port of a slave node to theprevious node (either slave or master) in the network. Since thetelegram will reach the master 36 prior to completing the entire networkring topology, in some embodiments a MTT (master tail telegram) isgenerated at the master and transmitted from the Q port of the master.This is done, e.g., to confirm a functional reverse path.

Naturally, other message routing schemes are possible in the currentinvention. For example, the direction of the routing (and thedesignations of the “previous slave node” and the “next slave node” arearbitrary). Message routing may also be changed in the operational phasein response to a fault in the network links and/or the slave nodes.

FIG. 23 illustrates a sample network with a ring topology with a faultbetween the slave nodes 32E and 34E. In the event that a link in thering network is severed or faulty, it is possible to maintain fulloperation of the network by re-routing the messages. In this case, thenetwork degenerates into a pair of chain topologies. As soon as themaster 36 fails to receive return telegrams that have been transmittedalong the entire ring (for example, an MDT sent from the P0 port of themaster 36 is not returned to the Q port of the master 36), the master 36can then transmit telegrams such as MST and the slave MDT's out of the Qport of the master 36. Depending upon the location of the fault, if anyof the slave nodes receive the MST at their P0 ports rather than their Qport, these slaves are configured to send their reply telegrams (e.g.,AT) through their P0 ports rather than their Q ports, as illustrated inFIG. 23. In this fashion, the network now automatically switches to anoperational topology of two slender trees rather than a ring. Moreover,the existence and location of the fault can be determined by notingwhich slaves do not transmit telegrams to the P0 port of the master 36(or conversely, which do transmit telegrams to the Q port of the master36). Naturally, the telegrams received at both ports can be compared todetermine if only a fault exists only at a single location. If the breakin the network chain occurs at one of the links, then the broken ringnetwork remains fully operational as two slender trees. In someapplications, it may be prudent to bring the machine to a safe condition(e.g., halt actuator motion) and effect repairs immediately.

In order to accomplish accurate co-ordinated control across a network,it is necessary to have some form of a network timer system. A networktimer system is also useful for the accurate scheduling of networktraffic.

Coordinated multi-axis motions are complicated by the propagation delaysof signals through the ring network topology. For example, reception ofan outbound telegram (e.g., MST or MDT) at the slave nodes will bedelayed by, e.g., propagation time through the cabling, the PHY's, andthe MAC repeaters of prior slave nodes. Therefore, networks that arephysically large or have many nodes on a ring (or have many tiers ontheir trees, as discussed herein later) experience the greatest delays.It is therefore desirable to introduce a correction factor to permit thesimultaneous application of the demands to the drives and to receivesimultaneously acquired feedback from the control entities throughoutthe network. The propagation delays are repeatable to a level ofuncertainty set by the PHY clock rates and accuracy. As described above,the delays can be measured during the discovery phase. There are twomain elements in the network timer system that can serve to compensatefor these propagation delays. The first is the strictly periodic MSTtelegrams emitted by the master, while the second is the local timecorrection circuitry present on each slave.

In a first embodiment, the master transmits two different correctionfactors to each slave related to the time taken for a telegram sent fromthe master to reach the slave in the forward direction and in thereverse direction around the ring. These quantities are called theforward and reverse path time correction or FPTC and RPTC, and theirtransmission can be performed, e.g., using a MCT during the discoveryphase 60.

During the operational phase 62, a MST is transmitted from the master 36on a periodic basis. The MST is the synchronization signal for theoperation of the motion control nodes on the network and, among otherfunctions, the MST helps synchronize the acquisition of position datafor each servo loop. Each node can phaselock an internal timer to theMST synchronization signal.

In one embodiment for operation with a complete network ring topology,since the MST is a continuously running master synchronization signal,each slave node can use its own FPTC as an offset when adjusting itsinternal timing. Thus, each node can be synchronized in time even whenthe FPTC is received at each node at different times. In effect, eachnode can anticipate receipt of the MST by the FPTC time interval. Thus,the internal timing of all of the nodes is based on simultaneoussynchronization, and position sampling and motion demand occursimultaneously, which is vital for multi axis coordination. The sameprocess can be used to offset the collection of data sent back to themaster with the RPTC delay for the transmitting slave node.

A second embodiment for operation in the event of a network faultcondition, the MST will be transmitted from both the P0 and the Q portof the master 36. In this case, unless the faulty link is immediatelyadjacent to the master 36, at least one slave node 36 will receive theMST at its P0 port rather than its Q port. As illustrated in FIG. 25, inthe case of a break between Slave 32E and Slave 34E, the MST that issent in the reverse direction is received by slaves 39E, 38E, and 34E.Thus, the FPTC is no longer appropriate for slaves 39E, 38E, and 34E,and those slaves must instead use the RPTC.

The benefit derived from the timing correction can be shown in thecontext of one embodiment of the invention illustrated in FIG. 26. Thenetwork controller (master 36) contains the motion planning function, adigital processor, which typically implements the position loop, as wellas other filtering and control functions. The network nodes (slaves 32Eand 34E) contain the motor drive(s), the position feedback interface,and the motion related I/O interfaces. The motion control network nodescan also include a processor that implements the current/torque controlloop. Both the slave nodes 32E and 34E and the master 36 will have anetwork interface.

As can be seen, all of the drive, motor, position feedback, and I/Osignals connect to the nodes. These nodes can be placed in closeproximity to the motors and sensors. The wiring to these nodes can bekept short. The network cable connects all of the nodes to the masterwith one thin cable. The speed and deterministic nature of the novelnetwork allow for centralized software control of all of the nodes andhence all of the axes in the motion control system. Yet the wiring isminimal compared to prior art unnetworked systems.

During the operational phase, a timing diagram such as that found inFIG. 27 can be used to illustrate the operation of a network such asthat illustrated in FIG. 26. A MST can be periodically transmitted bythe master 36 to synchronize the slave nodes of the motion controlsystem with the servo cycle clock (or a frequency related to the servocycle clock). The actual network transfer rate is many times faster thanthe frequency of the MST signal because much information must betransferred in each cycle. However, all network operations can be phaselocked to the MST signal. In one embodiment, the rate at which positionis updated in a motion control system is the servo cycle clock rate. Themotion trajectory for each axis is determined by the motion planningfunction of the master 36, and at each tick of the servo clock the nextposition demand, representing the next point on the motion trajectory,is presented to the drive at a slave node.

Although, in complex servo systems, many functions may be performed ineach cycle of the synchronization clock, a minimum of three operationsmust take place in each cycle. Firstly, the position of the actuatorand/or actuated object must be acquired. Secondly, this position datamust be transferred to the master 36. Thirdly, the velocity or torque orcurrent demands (based in whole or in part upon a feedback signalcalculated based on this or a previous clock cycle's position data) mustbe applied to the drive. In a centralized motion control system, thesefunctions are synchronized to each tick of the servo clock for eachaxis. This allows highly coordinated, multi-axis motion that iscontrolled by a centralized motion control system.

Referring in particular to the timing diagram of FIG. 27, theacquisition of position data at the slave nodes 32E and 34E fortransmittal back to the master control processor 36 is shown. Asillustrated, position data can be acquired simultaneously on two nodesusing the time correction function of the invention. As shown in FIG. 26for a two node system, the reception of the servo clock signal at slavenode 34E is delayed by an amount of time FPTC 1 and the reception of theservo clock signal at slave node 32E is delayed by FPTC 2. These delaytimes correspond to the propagation delay times for information passedfrom the master 36 to slave 34E and slave 32E, respectively. Thus, thereception of the servo clock signal at each slave node is at the samefrequency as the servo clock of the master 36, but offset in time. Sincethe respective FPTC has been previously stored in each slave nodeaccording to this embodiment of the invention, each node acquires thenext position update at the same real time (but at different timesrelative to the reception of the MST by that slave node).

The actuation of a velocity or torque or current demand by the drives ateach slave node is similarly accomplished (i.e., delayed by the suitableFPTC). Thus, it can be seen that the novel network retains centralizedmotion planning and position control while achieving absolutesynchronization with the servo clock of the master 36.

In addition to timing the activities that occur at the slave nodes, thetransmission of telegrams along the loop network must also be scheduledin the operational phase 62, e.g., to prevent telegram overlap. (Duringthe exemplary discovery phase 60 described above, scheduling of messagesis not a problem since the network is self-timed. The master 36transmits a message at any time and the slaves respond thereto shortlyafterwards).

During the operational phase 62 all messages are sent on a cyclic basis.In one embodiment, the master 36 transmits the MST on a periodic basis,and the MTT and the MDT's are sent at pre-defined offsets with respectto the start of the MST period. The slaves use their time-correctedlocal clock to emit their AT's at an offset determined and loaded intothe slave during the discovery phase 60.

Two exemplary scheduling schemes during the operational phase 62 are nowdescribed, namely strict timing and free timing. Scheduling is commonlydetermined by the master 36 and can be accomplished by setting thetiming parameters implemented in the slave nodes during the discoveryphase using, e.g., MCTs addressed to the individual slave nodes.

Free timing allows telegrams to be transmitted from a particular slavenode at a particular time relative to the MST synchronization signal.The telegrams can be transmitted in any predetermined order as long asthey are separated from one another by a sufficient “gap” (distance intime). With the system illustrated in FIG. 28, an example AT order couldbe AT4, gap, AT1, gap, AT3, gap, and AT2. The gap can be selected sothat the telegrams can travel in either direction around the ringwithout collision.

The absolute minimum gap in any timing scheme is stipulated by the PHYdevices (e.g., at least four IDLE symbols) to ensure, firstly, thesynchronization of the phase-lock loop timer within the PHY with the MSTand, secondly, to allow time for the re-synchronization of the scramblerwithin the PHY. In free timing, this gap must be respected under allconditions and commonly includes allowing for clock rate differencesbetween nodes the propagation time across the network (including allcabling and repeaters) from the farthest node. If the propagation timeis large enough and in the absence of this allowance, the head of AT4could collide with the tail of AT1.

Strict timing organizes the transmission times for the ATs to achievethe most efficient bandwidth utilization. In a simple ring network, thenodes send their ATs at a time that has been scheduled to follow (aftera suitable time gap for the PHY) after the AT from the previous node hasbeen transmitted to the master 36. The ATs are thus transmitted inorder, the node whose Q port is connected to the P0 port of the mastersends first and then indicates this transmission to the next node, andthe second node then transmits to the master 36 second, and so forth; inFIG. 22 this corresponds to 34E followed by 32E. Thus AT1 is sent first,followed by AT2, and so on. By using the above-described strict timingapproach, the minimum gap between telegrams stipulated by the PHYdevices is respected and collisions are inherently avoided. However, inthe presence of a fault condition, the direction of the AT messagesaround the ring will be reversed for at least some of the slave nodesand the transmission times set by the strict timing regime are notguaranteed to respect the proper inter-message timing gaps.

An alternate network topology is a full-duplex tree, as illustrated inFIG. 29. A full-duplex tree can be used where ring wiring isinconvenient or where a machine has an inherently modular structure andit is an advantage to attach a pre-wired sub-system without disturbingthe existing network wiring. Since a tree lends itself to machinery thatis modular, each module can be pre-wired and the modules can be added toa machine without disturbing the existing wiring.

In order to implement a tree, a slave node will, in the general case,have in addition to the P0 port, other ports designated P1, P2, . . . PN(310, 311). Each PN port will have a pair of FIFOs to allow it to repeattransmissions to and from the Q port (320).

In a preferred embodiment of the tree structure, a message received at aslave node PHY from the Q port is repeated to each of the ports P0, P1,P2 . . . PN, whereas a message received at the slave node PHY from anyPN port is only repeated through the PHY to the Q port. The repeaterfunction is thus bi-directional but messages sent from the masterfan-out throughout the tree whereas messages sent from a slave pass backto the master but, for simplicity's sake, do not fan out to the otherslaves. Each link of the tree structure runs from a PN connector to a Qconnector and there are no fundamental limitations on the number of PNconnectors, i.e. the fan-out or on the number of tiers to the tree. Inpractice, however, both fan-out and tree depth will be limited byeconomic concerns.

In a tree structure it may be of practical benefit in some embodimentsto have a slave node whose sole function is to increase the fan-out soas to reduce the depth of the tree, such a slave node could be arrangedto have no control entities and need not support an address. In thiscase, such a slave node would act as a repeating hub.

The discovery process in a tree is the same as for a ring but withcertain minor changes. The discovery process now fans out from eachslave node to as many other slave nodes as are attached. The MPTcontrols which of the PN ports of a given slave is active and thusallows selective configuration of the network tree, one node at a time.

Both strict and free timing as described above are applicable to a tree.The strict timing scheme is however somewhat different in a treenetwork; the AT's from the first tier (i.e. nearest the master) are sentsequentially and without overlap (at times set during the configurationphase by the master 36), and then those from the second tier, and soforth.

The reader will see that a network according to the present inventionlinks the elements of a motion control system that preserves theflexibility of the established centralized control model with all of thebenefits associated with networked connectivity that is applicable tothe multi-axis control system of machines of the highest performancelevel and largest physical size.

While the above description contains many specific details forimplementing such a network, these details should not be construed aslimitations on the scope of the invention, but rather as anexemplification thereof. Many other variations thereof are possible asobvious to one skilled in the art.

For example, a network according to the present invention could supportany of three techniques for time correction: no correction, staticcorrection and measured correction. Measured correction is the preferredtechnique described above, but an alternative that is simpler and lessexpensive to realize is static correction. Static correction is achievedby predetermining the delay of a given node and of the cable leading tothe node. The master node is then able to assign the slave a correctionfactor without any operational confirmation of this value. Correctionfactors for nodes father along the ring or tree are cumulative. Thestatic correction data could be a data sheet figure for the slave nodeor could be a calibrated value read previously measured for each deviceand read over the network. Moreover, in small networks, time correctioncould be dispensed with altogether.

Although the network may be configured so that the instant at which theMST leaves the master is aligned with the servo tick which in turn isaligned with the instant at which the slave nodes acquire position orother data and which in turn is also aligned with the instant at whichslave nodes give effect to new demand values, it is to be understoodthat these instants, while still simultaneous for all slaves, could beseparated by specified timing offsets and that this may be of practicaladvantage.

Subtracting the forward (or reverse) time correction parameters from thetime of reception of the MST is only one way of achieving a correctedtimer within the slave. Alternatively, the same effect can be achievedusing a phase-locked loop. In such a scheme, a “true local MST pulse” isgenerated by phase comparison between the “true local MST pulse” and thereceived MST delayed by the cycle time less the FPTC.

The preferred embodiment includes the discovery phase as describedabove. However, the network could alternatively—but far lessconveniently—be implemented without the discovery phase through thestatic assignment of node addresses and timing parameters using DIPswitches or other non-volatile memory in conjunction with a settingtool.

Moreover, it is possible to combine the functions of the exemplarytelegrams described above. For example, the data field within the MSTcould be modified to contain useful information. As another example, theMDT's described above carry data to one slave address. Alternatively, itis possible to use a single MDT and arrange for the operational data forall the slaves to be contained within this single MDT telegram.

In the above-described description, each control entity within a singlephysical slave node has a separate node address. Alternatively, wheremore than one control entity exists at a physical node, it is possible,in terms of both circuitry and network utilization, to send all of thecommand data in a single message for that node rather than as fourseparately addressed MDT's. Similarly, all feedback information can bebundled into a single AT.

In a previously described embodiment, the node statistics field isreserved for the purpose of indicating whether the last MST and MDT werereceived correctly and other network information. Alternatively, thenode statistics field could be located elsewhere in the AT or in afreestanding telegram devised for this purpose. Alternatively, the nodestatistics field can be omitted altogether.

The particular routing of messages in the tree topology previouslydescribed has been selected for its simplicity. Alternatively, the ATmessages could be reflected to all nodes. It is also possible to havethe messages circulate as on a ring, i.e. pass in and out of every node,even though the topology is a tree.

Naturally, the network can be based on a physical layer other than IEEE802.3-1998 Clauses 24-26, since there are no special requirements on thephysical layer. Any such layer supporting point-to-point communication,full or half-duplex (since full-duplex can be achieved using twohalf-duplex links), having a high bit-rate, a low inherent error rate,and good EMC properties could be used. One example of a physical layerthat can be used is the IEEE 802.3ab-1999 (‘Gigabit Ethernet overcopper’).

Various forms of computer readable media may be involved in storinginstructions for carrying out one or more sequences described herein.For example, the instructions may initially be carried on a magneticdisk that can be inserted into the master 36. The master 36 can thenload the instructions for implementing all or a portion of the presentinvention. Common forms of computer readable media include, for example,hard disks, floppy disks, tape, magneto-optical disks, PROMs (EPROM,EEPROM, Flash EPROM), DRAM, SRAM, SDRAM, or any other magnetic medium,analog or digital signals transmitted over a network connection, compactdisks (e.g., CD-ROM), or any other optical medium, punch cards, papertape, or other physical medium with patterns of holes, a carrier wave,or any other medium from which a computer can read.

Briefly recapitulating, a system for motion control, a method of using asystem for motion control, and a computer readable medium storinginstructions for operating a system for motion control according to thepresent invention provide a correction mechanism to account forpropagation delay. This system for motion control can includefull-duplex operation and/or strictly cyclic messaging. This system formotion control can also provide a bit rate that is high enough to updatethe torque/current demands of several drives at rates of 16 kHz or more.Moreover, this system for motion control can provide a choice betweenring or tree topologies, and between fiber or copper transmission media(or combinations thereof), and/or be inexpensive to implement andelectromagnetically compatible with different motion devices (e.g.,robust and able to operate over large ground potential differences).Moreover, this system for motion control can provide automatic systemconfiguration capabilities, automatic location of network faults,simultaneous application of demands at multiple drives, simultaneousacquisition of the data from multiple drives throughout the system,and/or the ability (in a ring-connected network) to operate fully in theevent of a severed link or similar fault.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise as specifically described herein.

1. A method of maintaining operation in the event of a fault of a systemfor motion control with a plurality of slaves in a ring networkcontrolled by a central controller, comprising: transmitting firstmessages addressed to plural respective slaves in a first directionalong said ring network; monitoring first reply messages transmitted bysaid plural slaves in a second direction along said ring network inresponse to the transmitted first messages; identifying, when firstreply messages are not received from each slave to which a first messagewas transmitted, a first subset of slaves from which said first replymessages are received, and based thereon, determining a second subset ofslaves exclusive of said first subset of slaves, from which respectivefirst reply messages were not received; transmitting in said seconddirection, when first reply messages are not received from each slave towhich a first message was transmitted, second messages addressed torespective slaves in said second subset of slaves; receiving secondreply messages transmitted by respective of said slaves of said secondsubset of slaves in response to said second messages, said second replymessages traveling in said first direction along said ring network;transmitting in said first direction third messages addressed to saidfirst subset of slaves; and transmitting in said second direction fourthmessages addressed to said second subset of slaves.
 2. The methodaccording to claim 1, wherein said ring network comprises a full-duplexring network.
 3. The method according to claim 2, further comprising: astep of reenumerating, from each of two ports of the central controller,two fragments of a ring that remain connected to the central controllerwhen one or more links of the full-duplex ring network no longerfunction or have been removed; and a step of operating the two fragmentsas two networks each having a tree topology.
 4. A system for operating aplurality of slaves controlled by a central controller via a ringnetwork in the event of a fault, comprising: means for transmittingfirst messages addressed to plural respective slaves in a firstdirection along said ring network; means for monitoring first replymessages transmitted by said plural slaves in a second direction alongsaid ring network in response to the first messages transmitted by saidmeans for transmitting first messages; means for identifying, when firstreply messages are not received from each slave to which a first messagewas transmitted, a first subset of slaves from which said first replymessages are received, and, based thereon, for determining a secondsubset of slaves exclusive of said first subset of slaves, from whichrespective first reply messages were not received; means fortransmitting in said second direction, when first reply messages are notreceived from each slave to which a first message was transmitted,second messages addressed to respective slaves in said second subset ofslaves; and means for receiving second reply messages transmitted byrespective of said slaves of said second subset of slaves in response tosaid second messages transmitted by said means for transmitting in saidsecond direction, said second reply messages traveling in said firstdirection along said ring network.